Garage door operators frequently are sold in the retail market. As such, it is important to provide them to the consumer in a way that makes them easy to install. Generally, two distinct types of door operators have been developed. One general type of operator available is a cable-operated or chain-operated door opener. Representative of these are U.S. Pat. No. 3,439,727 and U.S. Pat. No. 3,444,650. This invention relates primarily to the other general type, which type includes a longitudinally stationary, but rotatable screw which acts on a travelling or "partial" nut.
A door operator with single-piece screw, such as disclosed by U.S. Pat. No. 2,056,174, has been available for many years. Since more recently, a sectioned-screw arrangement has been available as seen from U.S. Pat. No. 4,352,585 that shows a door operator with sectioned screw parts disconnected at the time that the door operator is shipped to the customer. Also, U.S. Pat. No. 4,241,540 shows a folding screw arrangement in which the screw parts are interconnected by a coupling pivotably connecting each pair of screw parts.
U.S. Pat. No. 4,352,585, for example, provides a greatly improved coupling arrangement between individual screw sections. However, it does not address the screw-to-motor connection.
Up to now, in many screw drive door operators, the motor for driving the screw has included a hollow rotor which telescopically receives an end portion of the screw. Behind the motor, a clutch assembly provides the driving connection between the hollow rotor and the section of screw extending therethrough. The clutch could be adjustable to provide for varying the amount of torque transmitted from the motor rotor to the screw. Thus, in this type of door operator arrangement, the installer is required to insert the screw (or a section thereof) into the hollow rotor and then adjust the clutch to provide the proper amount of torque transmission from the motor to the screw.
Rear-mounted clutch assemblies may include a number of different elements to effect the driving connection between the screw and rotor, and therefore such rear-connection installation process can be complicated. If the rear clutch connection is made improperly, this could lead to noisy operation and also increase the chance of premature deterioration and even failure of the door operator. Further, the presence of a rear-connection clutch assembly significantly adds to the overall cost in manufacturing of a screw drive door operator. It also necessitates a hollow rotor assembly for the motor.
Whether the screw is front or rear connected, it is advantageous to include a device for absorbing longitudinal thrust occurring as a result of rotation of the screw in both directions. The screw undergoes substantial compression when the door opener is operated to lower the door because as the screw rotates to cause the slide or traveler to push against the stationary door in order to lower it, the inertia of the door exerts force back against the screw and therefore the motor. It is for this reason that U.S. Pat. No. 4,352,585 describes that the door operator has a motor shaft with bearings that include thrust bearings. The thrust bearings absorb the longitudinal thrust in both drive directions. The thrust absorption requirement presents a problem in developing a frontal connection between the screw and the motor, which frontal connection does not require a clutch.
Further, in both screw drive and chain drive operators, security against "back drive" of the door operator is of concern. "Back drive" is referred to as the condition where attempts are made at unauthorized opening of a garage door held closed by a door operator. During an attempt at unauthorized opening of the door, force exerted against the door is transmitted to the motor by either the screw in a screw drive operator, or the chain in a chain drive operator, and if sufficient, the transmitted force rotates the motor rotor in a direction opposite to the direction in which it rotated to close the door. In a screw drive operator, for example, when the door arm connects to the slide or traveler at substantially 90.degree. with respect to the longitudinal direction of the screw, and the screw is approximately horizontal, there is little concern that the motor could be back driven because all of the force exerted in attempting to unauthorizedly open the door is exerted perpendicular (in the Y-direction) to the screw. However, if the door arm and the section of screw intermediate the door arm and the door form an acute angle (for instance angle A in FIG. 1), there will be a force component along the axial direction of the screw (X-direction). If this force is sufficient, in the absence of a back drive preventing device, the force will urge the traveler to move back toward the motor, thus causing the screw to rotate. Rotation of the screw in turn causes the motor rotor to rotate. As the traveler moves rearwardly, the door arm rotates about a pivot point on the traveler such that the aforementioned angle becomes more acute and the force component in the longitudinal direction of the screw becomes greater. This force component, in the absence of a brake or other anti-back drive device for the motor, thus back drives the motor whereupon unauthorized entry can be accomplished.